As interior construction material or kitchen top and vanity material in construction site, various kinds of natural stones, engineered stone and the like (hereinafter, referred to simply as “grinding object material” or “stone material” depending on the situation) have been widely used. These stone materials as construction material are marketed as a slab plate 1800 mm–3000 mm on one side and the thickness is usually 20 mm, 30 mm or 40 mm which is presented by laminated in two layers 20 mm pieces and a slab plate composed of stone material used most generally has a thickness of 30 mm.
In recent years, the construction materials represented by these stone materials have been widely used not only for wall surface and floor surface of living space but also as interior materials of bath room and toilet because high grade appearance of its finish surface and excellent heat insulation are preferred. However, because these stone materials are extremely hard and brittle material and the edge corner acts just like a sharp edge blade, these edges which can make direct contact with the human being or the like are chamfered preliminarily to avoid a danger. For example, for the outdoor table and the like, a number of measures have been adopted to implement design esthetic preference as well as safety countermeasure by grinding the edges into a round configuration having a semi-circular section. The processing method for grinding the slab plate edge into a round configuration having semi-circular section is called full bullnose processing in this field, which has been known as one of special processing methods since before.
As a tool for implementing the full bullnose processing on the edge of a slab plate composed of stone material, so-called profile wheel (sometimes called “rounter bit”) in which ultra abrasive brazing layer for defining its grinding surface is formed into a configuration corresponding to the processing surface of a grinding object material, has been widely used. As for the profile wheel whose grinding surface is formed into a substantially semi-circular concave configuration, the grinding surfaces opposing each other are formed into a symmetrical configuration including the concave shape so that a pair of the profile wheels are defined by the two pieces or there is a single unit profile wheel in which the grinding surface in the concave shape having the substantially semi-circular section is formed integrally (see, for example, Catalogue of Gran Quartz co.).
Because the medium of the aforementioned stone material is of particular hard/brittle material, originally its processability is extremely bad and thus various devices have been taken since before to grind a grinding object material at a specified accuracy with a diamond tool or CBN tool fixed to the spindle of an electric or air tool rotating at high or low speeds. For example, there have been proposed a rotary grinding tool in which elastic body is interposed between a cutter blade and a spindle so as to absorb vibration or deflection of axis (see, for example Japanese Patent Application laid-open No. 2001-300855) and an offset type grinding tool in which a plurality of ultra abrasive brazing layers are formed on the outer periphery of its substrate and by forming and sintering the ultra abrasive brazing layer while pressurizing at least a portion near the outer periphery of a drawn substrate by means of dies, distortion of the substrate is removed by the sintering to reduce vibration originating from that distortion (see, for example Japanese Patent Application laid-open No. 2002-205274) and these have been actually realized.
It is not easy to fix the profile wheel on the spindle of a rotary electric or air tool such as a polisher in the same way as described above and carry out the so-called full bullnose processing on edges of a plurality of slab plates by manual operation to provide with a round shape having a semi-circular section by securing a specified curvature and a grinding distance Y. It is not sufficient to absorb a vibration or deflection of axis with the above-described method and even a skilled worker can hardly continue the full bullnose processing stably at a good accuracy. Thus, according to a conventional way, generally, slab plates subjected to the full bullnose processing by means of the profile wheels mounted on a special machine have been purchased in a necessary quantity and processed to a predetermined dimension on site and assembled together.